Sets a file as the proxy of this AVItem. Loads the specified file into a new FileSource object, sets this as the value of the proxySource attribute, and sets useProxy to true. It does not preserve the interpretation parameters, instead using the user preferences. If the file has an unlabeled alpha channel, and the user preference says to ask the user what to do, the method estimates the alpha interpretation, rather than asking the user.
This differs from setting a FootageItem’s main source, but both actions are performed as in the user interface.
fil e An ExtendScript File object for the file to be used as a proxy.